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In the 15 years since The Population Bomb was written, well over a billion people have been added to the human total. Amazingly, that number is greater than the entire population of the Earth at the time of the American Revolution.

Indeed, a short half-century ago there were just 2 billion people in the world. Now, there are 4.6 billion. In less than one brief lifetime, then, the human population of our planet has more than doubled.

Of course, such unprecedented growth might not be terribly serious were Earth not already overrun by people. In fact, today's human family is so large that it can survive only by behaving in a way no sane family would want to: by destroying its capital. The vast number of people now on this planet, you see, can be supported only by consuming or dispersing a one-time bonanza: our store of inherited wealth, which cannot be replenished on a time scale of any interest to society.

Therefore, one major concern of humanity must be to bring population growth to a halt as rapidly as is humanely possible, and then to start a gradual decline by keeping the birth rate (which is generally expressed as the number of people born annually per thousand people) slightly below the death rate. And that process must continue until a population is reached that can be supported indefinitely on renewable resources.

The size of the human population will depend upon, among other things, both the average level of affluence and the technology (and resources) used to support that affluence. However, it seems certain that, with foreseeable technologies and with a standard of living near that now enjoyed by the rich nations (and aspired to by the poor), our "optimum" population would be substantially smaller than today's — perhaps one billion or less.

Understanding Growth Rates

Of course, the human population is now moving away from an optimum level, and is actually increasing by some 75 million people annually. But there has been a cheering decline in birth rates in rich countries and, more important, in quite a few poor ones.

In the wealthy nations, growth rates have declined considerably. Some of these lands (such as Austria, East Germany, Sweden, and the United Kingdom) have in essence reached zero population growth (ZPG), and West Germany's population is gradually shrinking. Unfortunately, however, the rich nations with slow growth represent only about a quarter of humanity. And in the poor three-quarters, the numbers are still skyrocketing.

There are two reasons for these increases. First, birth rate decline, where significant, is still small in proportion to what is needed. For example, although India's birth rate has gone down from about 42 per thousand to 36 per thousand in the last decade or so, its target rate should be below 18. And in some areas, such as much of Africa, there hasn't been any significant decline in birth rates.

The second reason for our continuing population problems is the momentum built into expansion by virtue of the age composition of rapidly growing societies. These typically have a high proportion of young people — often 45% or more — under the age of 15. This means that, even after family sizes become small, the population will continue to increase for a long time.

How can this be? After all, once each couple has an average of just over two children, in effect (or so it would seem) simply replacing itself, shouldn't population growth stop? (The "just over" two children figure allows for those infants who die before reaching reproductive age.)

Sadly, the answer is no. ZPG occurs when birth rates and death rates are equal. Reducing a nation's reproduction rate to replacement level does not bring about ZPG immediately because of the inherent youth-heavy age composition of growing populations. There are, of course, many more young people to marry and have babies in such a population than there are elderly.

Thus, parents aged 20 to 30 may have only two children, but chances are they will still be alive, aged 40 to 50, when their children reach reproductive age. In this case the generations wouldbe just replacing themselves, but they would go on living after they reproduced, while the next generation would add more babies to the population. The death rate would not begin to rise significantly until one or two decades later, when people of that first replacement reproducer generation reached their 60's and 70's. And at that time, their grandchildren, the third generation, would be having their children.

By then, young people would make up a much less sizable proportion of the population than they did before, because families would have averaged a little over two children for four decades. Consequently, a relatively low number of couples would then be having the small families aimed for. And since birth rates would fall as death rates rose, they'd meet, some 60 years or so after replacement reproduction began, and population growth would finally come to a halt.

To give you some idea of the sheer magnitude of the momentum of population growth, consider the following: Suppose that a rapidly growing poor nation with an average family size of five or six children institutes a population control campaign and that — in about 30 years — the country manages to bring its family size down to replacement level. Under those circumstances, the population will continue to grow — as a result of the momentum provided by an initially youth-heavy population — for almost a century after the start of the campaign, and the final stabilized population will be on the order of 2.5 times that which existed when the control campaign began!

For example, suppose India, with a population in 1970 of some 540 million, managed to reach replacement reproduction around the year 2000. It would not stop growing until after 2050, and its final population would be almost 1.4 billion — or about the size of Planet Earth's entire human population in 1900.

Ten Billion Earthlings!

It's this momentum that led the World Bank in 1980 to project an end to wood population growth in about the year 2100 with roughly 10 billion souls. In this vision Kenya would have 109 million people instead of the current 16 million; Nigeria would increase from 85 to 425 million; Indonesia from 150 to 350 million; and Bangladesh from 93 to 314 million. Mexico would almost triple in size from 70 to 205 million.

We believe, however, that these projections are extremely optimistic in one way, and far too pessimistic in another. The optimism is a reflection of the complete ignorance of environmental constraints that pervades the World Bank's World Development Report, 1980. Should the planet proceed along the lines those predictions indicate, death rates would rise long before the population of Bangladesh reached 200 million, let alone 314 million. The implication that Earth can somehow sustain continued high rates of population growth until the middle of the next century indicates only the ecological naiveté of the World Bank.

On the other hand, the projections may be pessimistic about how soon replacement reproduction could be obtained. In most cases, this isn't presumed to occur in poor nations until nearly the middle of the next century. But China's experience indicates that it could be achieved much more quickly than that.

The World Bank forecasts are typical of those made by many such agencies in that they ignore the virtual certainty that death rates will rise in most nations in the not-too-distant future. In fact, increases have already been recorded, In Sri Lanka and parts of Bangladesh, for instance, the death rates rose in the late 1960's and early 1970's. They've also been increasing in some Latin American cities, and parts of Africa may be undergoing elevations in death rates now. Even the U.S.S.R.'s infant mortality rate has risen substantially in the last decade. And as increasing human numbers impose further stress on environmental systems, generally increasing death rates are, unfortunately, likely to take a stronger hand in controlling population growth.

The World Bank projections also ignore the possibility of population shrinkage as a result of low birth rates, even though a few nations such as West Germany have recently had declining populations, and China has made such a reduction a stated goal of its population policy.

It may still be possible to avoid catastrophe by moving the entire planet toward "negative population growth." Those who oppose achieving this objective through birth control are almost certainly promoting increases in the death rate. Remember that sooner or later, growth on a finite planet must end. The critical question is: "how"?

The World Development Report, 1980 was published for the World Bank by Oxford University Press. Information on death rate increase can be found in Signs of Change in Developing Country Mortality Trends: End of an Era? by Davidson R. Gwathkim Overseas Development Council Development Paper 30, February 1981.

Paul Ehrlich (Bing Professor of Population Studies and Professor of Biological Sciences, Stanford University) and Anne Ehrlich (Senior Research Associate, Department of Biological Sciences, Stanford) are familiar names to ecologists and environmentalists everywhere. But while most folks are aware of the Ehrlichs' popular writing in the areas of ecology and overpopulation (most of us — for instance — have read Paul's book The Population Bomb), few people have any idea of how deeply the Ehrlichs are involved in ecological research (the type that tends to be published only in technical journals and college texts). That's why we're pleased to present this regular semi-technical column by these well-known authors/ecologists/educators.

The Ehrlichs' work is supported in part by grant from the Koret Foundation of SanFrancisco.

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